1. Field of the Invention
The present invention relates to an automobile antenna system and, more particularly, to an improved automobile antenna system for effectively detecting broadcast waves received by the automobile body and then supplying signals thus detected to various kinds of receivers mounted in the automobile.
2. Description of the Prior Art
Antenna systems are indispensable for automobiles required to positively receive various broadcast and communication waves such as those for radio, television or telephone by using receivers mounted in the automobile. These types of antenna systems are also important in allowing communication to take place between automobiles and other stations, for example, transmitting and receiving citizen band radio waves. Therefore, such antenna systems play a major role in communication functions which will henceforth be regularly incorporated in automobiles.
One conventional type of common antenna system is known as a pole antenna. Although the pole antenna which projects outwardly from the body of an automobile exhibits desirable performance in terms of reception, it has always been the fate of the pole antenna to be treated as a nuisance from the viewpoint of vehicle body design.
Furthermore, the pole antenna involves various problems in that it is exposed to damage such as breakage or bending in actual service and in that it becomes the target of mischief or theft and, additionally, it generates unpleasant noise during high-speed running. For these reasons, there has heretofore been a strong demand for eliminating such pole antennas.
In particular, since frequency bands for broadcast or communication waves received in the interior of an antomobile have recently been widened, it is necessary to install a plurality of antennas corresponding to each of the frequency bands received. Consequently, the installation of a plurality of antennas involves disadvantage in that the aesthetic appearance of an automobile is spoiled and the reception performance of the various antennas is remarkably deteriorated due to the mutual electrical inteference caused therebetween.
Several efforts have previously been made to replace the above-mentioned pole antenna system or to conceal the system from the exterior. As an example, a means of applying a length of antenna wire to the rear windshield of an automobile has been put into practical use.
As another conventional means of solving the above-noted problems, proposals have been made to detect surface currents which are induced by broadcast waves on the vehicle body itself. Although it is considered that, seemingly, the utilization of currents flowing on the vehicle body might be expected to be the most positive and efficient means of reception, the experiments carried out to date have shown disappointing results.
One of the reasons why the surface currents induced on a common vehicle body by broadcast waves have not been utilized well is that the values of the surface currents are not so large as was expected. Although the prior art mainly uses surface currents induced on the roof plate of the vehicle, it is still impossible to obtain a detected output showing a utilizable level.
Another reason is that interference at a high level of noise is mixed in the surface currents. Such noise is mainly generated by the engine ignition system and the battery charging regulator system. As long as the engine is operating, such noise continues to leak into the vehicle body, thus preventing any clear reception of broadcast waves at a practicable level.
Several proposals have heretofore been made in an attempt to cope with these adverse conditions. A conventional type of antenna system using currents induced on the vehicle body by broadcast waves is disclosed in Japanese Patent Publication No. 22418/1978 in which electrical insulation is formed at a portion of the vehicle body on which electric currents are concentrated and the currents flowing between the opposite ends of the insulation are directly detected by a sensor. It is true that this conventional antenna system suggests that it can provide a detected signal of a utilizable level which is superior in S/N ratio. However, since a pickup structure therefor requires a cutout in a portion of the vehicle body, it cannot be applied to normal mass-production types of automobiles.
Another conventional antenna system is disclosed in Japanese Utility Model Publication No. 34826/1978 in which an antenna including a pickup coil for detecting a current flowing in the pillar of a vehicle body is proposed. This prior art was useful in anticipating the course of development of systems based on incorporation of an antenna into a vehicle body. However, it is of no practical use for the pickup coil to be located in the vicinity of a pillar in a direction perpendicular to the longitudinal axis thereof. In addition, since this pickup arrangement is not capable of providing any antenna output of utilizable level, it has been regarded merely as a casual idea.
As described above, the prior-art antenna systems have not necessarily been successful in detecting currents induced on the vehicle body by broadcast waves.
In particular, no solution has heretofore been found to various problems which arise with a pickup structure for effectively detecting currents induced by broadcast waves which are conducted by the vehicle body and a pickup arrangement capable of obtaining a utilizable S/N ratio. Rather, the results of various kinds of experiments have suggested that it might be theoretically impossible to use an antenna system which utilizes currents flowing on the vehicle body.